1. Field of the Invention:
The present invention relates to an electrically operated power steering system having an electric motor for producing assistive forces to assist manual steering forces in steering a motor vehicle.
2. Description of the Prior Art:
There have heretofore been known electrically operated power steering systems for controlling an assistive torque supplied from an electric motor based on a manual steering torque that has been detected by a steering torque detector. Examples of such electrically operated power steering systems are disclosed in U.S. Pat. Nos. 4,657,103 and 4,681,183.
However, since the conventional electrically operated power steering systems control the assistive torque based on the detected manual steering torque, it has been difficult for the power steering systems to take into account various factors that impair the driver's feel as to steering responses, i.e., the inertia of the electric motor and the friction of the power steering mechanism.
Another problem with the conventional electrically operated power steering systems is that forces tending to return the steering wheel are weak in the vicinity of the neutral position of the steering wheel because the self-aligning torque of the tires is small. Therefore, when the steering wheel is in the vicinity of the neutral position, the steering wheel is apt to be forcibly turned by road surface irregularities or undulations.